1. Field of the Invention
The present invention relates to a field effect transistor comprised of a nitride based compound to be used for a device for power electronics and a device of high frequency amplification, and to a process for producing the same.
2. Description of the Related Art
A wide band gap semiconductor representative of a III-V group nitride based compound is extremely attractive as a material for a semiconductor device for a high temperature, a large power, or for a high frequency, because of having a breakdown voltage as high, a electronic transport property as satisfactory, and a thermal conductivity as satisfactory. Moreover, regarding a field effect transistor (an FET) having an AlGaN/GaN hetero structure for example, a two dimensional electron gas is generated in an interface due to a piezoelectric effect. The two dimensional electron gas draws attention because of high electron mobility and carrier density. Further, a hetero-junction FET (an HFET) using the AlGaN/GaN hetero structure has a low resistance and a fast switching speed, so that it is possible to perform an operation at a high temperature environment. The features are suitable for an application of a power switching.
The ordinary AlGaN/GaN HFET is a device of a normally on type, in which an electric current flows in a case where a bias is not applied to a gate, and then an electric current is cut off by applying a negative electric potential to the gate therein. In the application of the power switching, for securing safety, it is preferable to use the device of the normally off type, in which an electric current does not flow in the case where a bias is not applied to a gate, and then an electric current flows by applying a positive electric potential to the gate.
In order to produce the device of normally off type, it is necessary to adopt an MOS structure. For example, International Patent Application Publication No. 2003/071607 has disclosed a field effect transistor having the MOS structure (an MOSFET), wherein a carrier supplying layer comprised of AlGaN or the like is etched off at a gate, and an insulating layer is formed on an etched surface of a carrier drifting layer.
In the field effect transistor disclosed in the patent document, a side wall of the carrier supplying layer thus etched off is formed approximately vertical to the etched surface of the carrier drifting layer. As a result, an electric field is converged at a corner with a right angle formed with the side wall of the carrier supplying layer and the etched surface of the carrier drifting layer between the gate and a drain, thereby decreasing a breakdown voltage.